The world's aquifers are being used faster than they can be replenished and, in some cases, at rates that have more than doubled since the 1960s. It is mostly agricultural irrigation that is driving the increase, because it accounts for 70 to 80 percent of global groundwater usage.

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An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt, or clay) from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology. Some examples of major aquifers follow:

The Great Artesian Basin situated in Australia is arguably the largest groundwater aquifer in the world. It plays a large part in water supplies for Queensland and remote parts of South Australia.

The Guarani Aquifer is shared by Brazil, Argentina, Paraguay and Uruguay.

The Ogallala Aquifer of the central United States is one of the world's great aquifers, but in places it is being rapidly depleted by growing municipal use, and continuing agricultural use. This huge aquifer, which underlies portions of eight states, contains primarily fossil water from the time of the last glaciation. Annual recharge, in the more arid parts of the aquifer, is estimated to total only about 10 percent of annual withdrawals.

An example of a significant and sustainable carbonate aquifer is the Edwards Aquifer in central Texas. This carbonate aquifer has historically been providing high quality water for nearly 2 million people, and even today, is completely full because of tremendous recharge from a number of area streams, rivers and lakes.

A team led by Marc Bierkens of the Utrecht University and the International Groundwater Resources Assessment Center at Deltares in the Netherlands recently used a model of water flow across the landscape to estimate how much water soaks into, runs off of or evaporates from parcels of land across the globe.

They combined this with the best available information on groundwater usage to calculate the net amount of water leaving or returning to aquifers.

Certain areas emerged as potential zones of groundwater depletion: northeastern China, northwestern India, Iran, northeastern Pakistan, southeastern Spain, the central United States, California's Central Valley and Yemen, findings that are consistent with local studies of these regions, the authors said.

The team published their findings in Geophysical Research Letters.

While over a third of the world's population suffers from water stress, the most important consequences of overusing groundwater will be in agriculture, where most groundwater is used.

"A lot of this is abstracted by small farmers with small wells," Bierkens said. "It doesn't mean that the water is depleted immediately, but it could mean that the small farms wouldn't be able to reach it anymore."

"If you run out of water you cannot grow crops anymore," he added. "You are basically extending your crop production on borrowed water."

Slowing groundwater use will require using more sophisticated irrigation techniques that use less water, developing crop types that can survive on lower quantities of water, and redirecting water on the landscape so that a higher proportion soaks back in to replenish the groundwater.

Aquifer Storage and Recovery is the re-injection of potable water back into an aquifer for later recovery and use. This has been done for municipal, industry and agriculture use. The first agriculture recovery wells were put into service in Oregon in the autumn of 2006.

"We've known that groundwater depletion is becoming more and more of an issue," said Matthew Rodell of NASA Goddard Space Flight Center in Greenbelt, Md., who has used satellite data to come to similar conclusions about groundwater usage.

For further information: http://news.discovery.com/earth/groundwater-aquifers-agriculture-irrigation.html#mkcpgn=rssnws1